1. Field of the Invention
This invention relates to visual image processing and, in particular, to adaptive filtering of a visual image. Most particularly, the invention relates to adaptively filtering a target version of a visual image produced by processing an original version of the visual image, where the adaptation of the filtering is based on one or more characteristics of the original visual image (the control image).
2. Related Art
In visual image processing, it is often necessary or desirable to filter a visual image to remove undesirable artifacts (e.g., adaptive-gain noise, upsampling artifacts, compression artifacts) from the image. Such artifacts may be present in a visual image as originally obtained or can be introduced into a visual image by previous processing of the image. For example, a decompressed visual image produced from an MPEG or JPEG compressed image can include blocking artifacts and intrablock directional noise introduced by the compression. Or, for example, an upsampled visual image can include blocking artifacts introduced by an upsampling filter. Or, for example, a visual image that has undergone brightness equalization can include pixel noise in high gain areas of the processed image.
An adaptive filter can be used to mitigate artifacts in a visual image that have been introduced by previous processing of the visual image. In an adaptive filter, the operation of the filter can vary in accordance with a characteristic of a version of the visual image being filtered. Herein, the version of a visual image used to control the adaptation of an adaptive filter used to filter the image is referred to as the “control” version of the visual image or the “control image.”
An adaptive filter has been implemented so that the adaptive filter is directionally adaptive (i.e., the orientation of the filter with respect to the visual image can change). However, as discussed in more detail below, it can be advantageous for an adaptive filter to have other adaptation characteristics.
Previously, the adaptation of an adaptive filter has been controlled in accordance with a characteristic of the version of the visual image being filtered (which is sometimes referred to herein as the “target” version of the visual image or the “target image”), i.e., the target image is the control image. However, artifacts introduced into the target version of the visual image by previous processing of the visual image can adversely affect the adaptive filtering. For example, if the target image is an upsampled visual image, the directionality of an adaptive filter applied to the target image can be adversely affected by blocking artifacts introduced into the target image by the upsampling (this is particularly so when a simple upsampling method is used, such as pixel replication). As discussed in more detail below, it can be advantageous for the adaptation of an adaptive filter to be dependent on a version of a visual image other than the target version of the visual image.